1. Field of the Invention
Azabicyclic, azatricyclic and azaspirocyclic derivatives of aminocyclohexanes which are systemically-active as NMDA, 5HT3, and nicotinic receptor antagonists, pharmaceutical compositions comprising the same, method of preparation thereof, and method of treating CNS disorders which involve disturbances of glutamatergic, serotoninergic, and nicotinic transmission therewith, for treating immunomodulatory disorders, and for treating infectious diseases.
2. Prior Art
NMDA Antagonists
Antagonism of glutamate receptors of the N-methyl-D-aspartate (NMDA) type has a potentially wide range of therapeutic applications [19]. Functional inhibition of NMDA receptors can be achieved through actions at different recognition sites such as the primary transmitter site, strychnine-insensitive glycine site (glycineB), polyamine site, and phencyclidine site located inside the cation channel. The NMDA receptor channel blockers act in an uncompetitive “use-dependent” manner, meaning that they usually only block the channel in the open state. This use-dependence has been interpreted by many to mean that stronger activation of the receptor should lead to a greater degree of antagonism. Such a mode of action has further been taken to imply that this class of antagonist may be particularly useful when overactivation of NMDA receptors can be expected, such as in epilepsy, ischaemia, and trauma. However, initial clinical experience with the selective, high affinity, strongly use-dependent uncompetitive NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate ((+)-MK-801) has been disappointing. Namely, therapeutic efficacy in epilepsy was poor while some psychotropic side effects were apparent at therapeutic doses. These observations, together with the fact that phencyclidine abusers experience similar psychotropic symptoms, has led to the conclusion that uncompetitive antagonism of NMDA receptors may not be a promising therapeutic approach.
However, the use of more elaborate electrophysiological methods indicates that there is no equality between different uncompetitive antagonists since factors such as the speed of receptor blockade (on-off kinetics) and the voltage-dependence of this effect may determine the pharmacodynamic features in vivo, i.e., therapeutic safety as well. Paradoxically, agents with low to moderate, rather than high, affinity may be desirable. Such findings triggered a reconsideration of the concept of uncompetitive antagonism of NMDA receptors in drug development [19, 22]. Uncompetitive NMDA receptor antagonists, such as amantadine and memantine—which fulfill the above criteria—have been used clinically for several years in the treatment of Parkinson's disease and dementia respectively, and do indeed rarely produce side effects at the therapeutic doses used in their respective indications.
In view of the above mentioned evidence, we have developed a series of novel uncompetitive NMDA receptor antagonists based on the azabicyclic, azatricyclic and azaspirocyclic aminocyclohexane structure.
5-HT3 Receptor Antagonists
5-HT3 receptors are ligand gated ionotropic receptors permeable for cations. In man 5-HT3 receptors show the highest density on enterochromaffin cells in the gastrointestinal mucosa, which are innervated by vagal afferents and the area postrema of the brain stem, which forms the chemoreceptor trigger zone.
Since 5-HT3 receptors not only have a high density in the area postrema but also in the hippocampal and amygdala region of the limbic system, it has been suggested that 5-HT3 selective antagonists may have psychotropic effects (Greenshaw & Silverstone, 1997).
Indeed, early animal studies suggested that the 5-HT3 receptor antagonists, in addition to their well recognized anti-emetic use, may well be clinically useful in a number of areas. These include anxiety disorders, schizophrenia, drug and alcohol abuse disorders, depressive disorders, cognitive disorders, Alzheimer's disease, cerebellar tremor, Parkinson's disease treatment-related psychosis, pain (migraine and irritable bowel syndrome), and appetite disorders.
Neuronal Nicotinic Receptor Antagonists
At present, ten alpha subunits (alpha 1-10) and four beta subunits (beta 1-4) for nicotinic receptors are known. α4β2 receptors are probably the most common in the CNS, especially in the hippocampus and striatum. They form non-selective cation channels with slowly, incompletely desensitizing currents (type II). Homomeric α7 receptors are both pre- and postsynaptic and are found in the hippocampus, motor cortex and limbic system as well as in the peripheral autonomic nervous system. These receptors are characterized by their high Ca2+ permeability and fast, strongly desensitizing responses (type 1A). Changes in nicotinic receptors have been implicated in a number of diseases. These include Alzheimer's disease, Parkinson's disease, Tourette's syndrome, schizophrenia, drug abuse, nicotine abuse, and pain.
Based on the observation that the nicotinic agonist nicotine itself seems to have beneficial effects, drug development so far is aimed at the discovery of selective nicotinic agonists.
On the other hand, it is unclear whether the effects of nicotinic agonists in, e.g., Tourette's syndrome and schizophrenia, are due to activation or inactivation/desensitization of neuronal nicotinic receptors.
The effects of agonists on neuronal nicotinic receptors is strongly dependent on the exposure period. Rapid reversible desensitization occurs in milliseconds, rundown occurs in seconds, irreversible inactivation of α4β2 and α7 containing receptors occurs in hours and their upregulation occurs within days.
In other words, the effects of nicotinic “agonists” may in fact be due to partial agonism, inactivation and/or desensitization of neuronal nicotinic receptors. In turn, moderate concentrations of neuronal nicotinic receptor channel blockers could produce the same effects as reported for nicotinic agonists in the above mentioned indications.